1. Field of the Invention
The present invention relates to chemical dyes which can be used as fluorescent markers. The present invention particularly relates to fluorescent monomethine cyanine-based compounds which have been sterically rigidized by the inclusion of a bridging two-carbon alkyl group between the compound's heterocyclic groups and which may be chemically modified to include chemical moieties to allow the compounds to covalently or noncovalently associate with a material to thereby impart fluorescent properties to the material.
2. Description of the Invention Background
Fluorescent dyes are generally known for imparting fluorescence to biological and nonbiological materials and have been used to detect various biological or other materials by procedures such as fluorescence microscopy, fluorescence immunoassay techniques, and flow cytometry. The primary advantages of fluorescent dyes compared with dyes detectable only through light absorption include (i) the emission of light by fluorescent dyes at a wavelength different from their excitation wavelength; (ii) the greatly enhanced detectability of fluorescence emission compared to light absorption; and (iii) the generally minimal level of background fluorescence in most biological materials.
A common method for labeling biological and nonbiological materials with fluorescent dyes is to create a fluorescent complex through covalent bonding between groups on the dye molecules and compatible groups on the material. In this way, materials such as, for example, cells, tissues, amino acids, proteins, antibodies, enzymes, drugs, hormones, nucleotides, nucleic acids, polysaccharide, and lipids may be chemically labeled and quantified or may be used as fluorescent probes that bind specifically to target materials (genetic sequences, haptens, antibodies, analytes, etc.) that are to be detected by fluorescence methods. Polymer particles, cells, and other materials labeled with fluorescent dyes can also be used as fluorescent standards in flow cytometers, imaging microscopes, and other fluorescence-based detection equipment. Minute fluorescent polymer particles can also be used as labels in immunofluorescence tests, toxicology testing, and analysis of genetic sequences. Because of their utility in research and medicine, a large market has developed for fluorescent reagents including prepared protein, antibody, and nucleic acid, and other probes pre-labeled with detectable fluorescent or fluorescent dye compounds.
Because most dye molecules are either nonfluorescent or only weakly fluorescent, available fluorescent dye markers have been derived from a relatively limited number of fluorescent aromatic structures. New fluorophores with optimal properties are rarely developed. Two common classes of fluorescent dyes are those derived from the fluorescein and rhodamine chromophores. Fluoresceins fluoresce green light whereas rhodamines fluoresce in the green-orange and red regions of the spectrum. The rhodamines are difficult labeling reagents to use, are not particularly fluorescent when bound to proteins, and often cause the precipitation of the labeled protein when the dye-to-protein ratio is greater than 2:1. One particular fluorescein dye, fluorescein isothiocyanate ("FITC"), and its conjugates, enjoy wide acceptance primarily because they have a relatively high extinction coefficient and have a high quantum yield. (Quantum yield is generally related to a molecule's rigidity or planarity and indicates the molecule's propensity to fluoresce, i.e., give off energy as light, rather than give off heat when energy is provided to the molecule.) However, fluorescein dyes have a number of disadvantages, including their strong tendency to photobleach when illuminated by a strong excitation source such as the lamps used in fluorescence microscopes. When a fluorescent compound photobleaches, a large percentage of the compound's fluorescence may be lost within seconds of illumination, resulting in a rapidly diminishing image. Also, when performing fluorescence assays, the loss of image through time by photobleaching makes quantifying results much more difficult and will ultimately result in a decreased ability to detect the analyte. Reagents, such as propyl gallate and p-phenylene-diamine, may retard but do not entirely eliminate photobleaching. The fluoresceins also have a pH-sensitive absorption spectrum and fluorescence yield decreases below pH 8 and the fluoresceins do not fluoresce at low pH.
Multiple fluorophores of different colors are used simultaneously in multi-parameter analyses for detecting and correlating different fluorescently-labeled materials in such procedures as flow cytometry, microscopy, chromatography and various other detection systems. In multi-parameter analyses, a number of fluorescent compounds having a binding affinity for different targets and having different maximum emission wavelengths are used to detect and quantify the sample's various targets. To reduce the overlap of fluorescence signals in multi-parameter analyses that are emitted from the target materials labeled with different fluorescent compounds, it is desirable to use fluorescent compounds with narrow absorption and emission bands.
One class of blue-fluorescing dyes, the coumarins, suffer from a number of disadvantages. For example, the coumarin-based fluorophore 7-amino-4-methylcoumarin acetate has broad absorption and emission peaks, Also, this compound has a relatively low extinction coefficient of approximately 17,000 l/mol-cm (Handbook of Fluorescent Probes and Research Chemicals, Molecular Probes Inc., Eugene, Ore.), thereby providing a fluorescence capacity (equal to the mathematical product of the extinction coefficient and the quantum yield) of approximately one-quarter that of the present inventions' compounds. Similarly, a fluorescent labeling dye available from Molecular Probes, Inc., under the trade name Cascade Blue (having the structure shown below where R is a reactive group) has an extinction coefficient of only 29,000 l/mol-cm. ##STR2##
The cyanine compounds are now recognized as fluorescent labeling dyes. Cyanines generally include two heterocyclic groups connected by a chain of conjugated double bonds with an odd number of carbon atoms and have been used as spectral sensitizers for photographic film. Cyanine compounds are utilized as spectral sensitizers in, for example, U.S. Pat. Nos. 4,337,063 (Miraha et al.) and 4,404,289 (Masuda et al.), 4,405,711 (Masuda et al.), and British Patent No. 1,529,202 (Exekial et al.). Fluorescence is not necessary for the photographic applications in those patents and fluorescent properties are not mentioned in those patents. The utility of cyanine compounds as fluorescent dyes was discovered only recently.
Cyanine compounds known to be useful fluorescent dyes include the unrigidized, arylsulfonated cyanine compounds of U.S. Pat. No. 5,268,486 to Waggoner et al, having the following general structure: ##STR3## wherein, the dotted lines represent one to three rings having five to six atoms in each ring. R.sub.3, R.sub.4, R.sub.8 and R.sub.9 groups are attached to the rings. At least one of the R.sub.8 and R.sub.9 groups is a sulfonic acid or sulfonate group and at least one of the R.sub.1, R.sub.2, R.sub.3, R.sub.4 and R.sub.7 groups is a moiety that will react with amino, hydroxy, phosphoryl, or sulfhydryl groups. The Waggoner et al. patent relates to trimethine, pentamethine, etc., cyanines (i.e., cyanines with more than a single double bond in the conjugated chain) and which fluoresce in the green, orange, red and near-infrared regions of the spectrum. Cyanines of this type have not been considered useful covalent labels.
One class of the cyanines includes a single atom bridging the heterocycles. These compounds are referred to herein as "rigidized" cyanines because the bridging atom restricts movement of the heterocycles about the conjugated carbon atom chain. Certain rigidized cyanines have been developed for photographic sensitization. See U.S. Pat. Nos. 2,541,400 to Brooker et al. and 3,148,187 to Heseltine. These cyanines cannot be used as fluorescent labeling dyes.
A class of monomethine cyanine-type compounds having a single atom of boron bridging the cyanine's heterocyclic groups is disclosed as useful fluorescent marker compounds in pending U.S. patent application Ser. No. 08/474,056 entitled "Rigidized Monomethine Cyanines" filed on Jun. 7, 1995, the entire disclosure of which is hereby incorporated herein by reference.
Considering the above, but for the compounds disclosed in the above pending application, the existing compounds recognized as fluorescent markers suffer from several disadvantages. Also, although certain rigidized cyanine complexes are known, the majority have not been evaluated for their suitability as fluorescent dye markers.
Accordingly, it is an object of the present invention to provide bright, highly fluorescent, strongly light absorbing dyes that can be used to covalently or noncovalently associate with a material to impart fluorescent properties to the material.
It is also an object of the invention to provide fluorescent markers having high quantum yields and extinction coefficients, and therefore high fluorescence, compared with available fluorescent marker compounds, and that are relativity insensitive to pH.
It is a further object of the present invention to provide fluorescent markers that are relatively photostable, have sharp and distinct absorption and emission maxima, and have relatively small stokes shifts.
It is also an object of the present invention to provide fluorescent compounds having a short resonance distance relative to the existing fluorescent cyanine compounds so that the compounds of the present invention may have maximum absorptive and/or emissive wavelengths in the blue spectral region below 500 nm.
An additional object of the present invention is to provide a fluorescent compound having a general chemical structure that is easily modified by the addition or substitution of chemical moieties to, for example, modify the compound's emissive and absorptive maxima and solubility in polar and nonpolar media, change the reactivity of the compound, and covalently or noncovalently associate the compound with a material.